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We just finished a major over-haul of our A&DR 700 comp and 760 compex units. This was not just a tune-up and repair, we went through them thoroughly and found some very interesting things that I thought might be valuable to others.
So far, we've seen 4 different schemes for the compexes, 2 for the F700s. Evolution of the designs includes things like extra trim pots for the compression curve, attempts to tame FET gain differences and on-board v regulation.
Here are the high spots, but feel free to ask questions.
- Current draw from pot wipers causes difficulty in adjustments and eventual
failure of the trim pot. Generally a current draw of no more than 20
microamperes DC should be drawn from a potentiometer wiper to avoid this
although pot manufacturer specifications vary on this point. This shows up
in several pots: compression curve, OS bias (F700), meter range, expander
curve. There are more suspect places but if an old pot has a 'dead spot'
then, wiper current draw is a prime suspect. This was not really well known
when these compressors were designed, but it is a problem that we know
- Drift in zener diode voltages causes some matching issues. Main power
supply might want to use a bandgap reference instead of a zener diode. Also,
I would stay away from Diodes, Inc. zeners and use Vishay or maybe NXP. Not
all zeners are created equally. We changed them all.
- 'inverter' stage setting is very twitchy and probably should not be. The
pot probably should be split up into three resistors to make the pot less
sensitive - the actual trim range should not be that high. We made this mod
with one of our units with good results.
- The control voltage pot of the 760 could be replaced with a shaping
network like found on the later F700 PC board to help linearize the FET gain
law. The risk here is that the F700 configuration would draw significant
wiper current. Adding buffers (just a 2N4401 or something like that) after
each pot might help. Considering this one for our "frankenstein" unit.
- On the F700 the output stage bias current setting is a bit problematic as
when the wiper goes open circuit the stage blows up. Four series diodes work
much better in this position, or perhaps a fixed resistor. We prefer the
diodes. 4 of our vintage units had previous damage here.
- For gain curving adjustments, the compressor should be placed in 1:1 mode
(no detector activity) and with an external CV applied to trim the FET bias and
compression curve for light compression matching, then the parallel compression
curve stages for further gain reduction in stages. Our feeling is that three breakpoints
should be sufficient. Once the compression curve is set with the external CV, the
internal detector gains should be tight enough and only the detector output
should need adjustment. The units would all be stereo matched(mix-and-match)
at this point.
- 1% resistors, please.
- no electrolytics in the compression time constant circuit. These get leaky
and cause the compressor to behave very strangely.
- plan on replacing the input and output pots. ALL of ours were shot or very dicey.
- There's no good way to linearize the threshold control POT on the 760 expander. We
used a type 01 12 way Elma switch in there. Works great and gives a quick match in stereo.
- We managed a .5 db match for pairs in the 6 Helios desk units. This includes CV links
for the 760 XN Expanders. NO attempt to pre-match FETs.
- If there was such a thing as a very compact PC mount multi-turn trimmer, I'd use them
in the curve trims. TOUCHY, even with new pots.
- Contrary to popular belief, there IS a trim for the limiter threshold, it's internal. Tempting
to bring it to the panel, but it's already very crowded.
The sound of the compressor is very nice, I'd put it up there alongside any
of the 'great compressors'. Once adjusted properly, they sound great. It
seems, though, that many problems are attributed to current draw from the
trimpot wipers and twitchy adjustments, as well as the sometimes too compact
layout of the F760's used on the Helios console.